1. Field of the Invention
The present invention relates to a semiconductor device testing apparatus suitable for testing one or more semiconductor devices, particularly one or more semiconductor integrated circuit elements (as will be referred to as IC or ICs hereinafter) which are typical examples of the semiconductor devices. More particularly, the present invention relates to a semiconductor device testing apparatus of the type in which semiconductor devices to be tested are transported, for testing, to a test or testing section where they are brought into electrical contact with a tester head (a component of the testing apparatus for applying and receiving various electrical signals for testing) to perform an electrical test of the semiconductor devices, followed by being carried out of the test section and then the tested semiconductor devices are sorted out into conformable or pass articles and unconformable or failure articles on the basis of the test results, and a semiconductor device testing system having a plurality of such semiconductor device testing apparatus.
2. Description of the Related Art
Many of semiconductor device testing apparatus (commonly called IC tester) for applying a test signal of a predetermined pattern to a semiconductor device to be tested, i.e. device under test (commonly called DUT) and measuring the electrical characteristics of the devices, have a semiconductor device transporting and handling or processing apparatus (commonly called handler) mounted thereto which transports semiconductor devices to a test section, brings them into electrical contact with a tester head in the test section, after the testing, carries the tested semiconductor devices out of the test section, and sorts them out into pass articles and failure articles on the basis of the test results. In the specification, the testing apparatus which comprises a combination of the IC tester and the handler mounted or connected thereto of the type described above is termed "semiconductor device testing apparatus". In the following disclosure the present invention will be described by taking ICs typical of semiconductor devices for example for clarity of explanation.
As the density of elements integrated on a semiconductor substrate or chip in an IC becomes higher, the number of terminals or pins of the IC is increased, and it is difficult to test such an IC having a large number of terminals using an IC testing apparatus having a naturally dropping type handler mounted thereto in which ICs are caused to slide down in a sloped carrying path or groove by their gravities for testing the ICs. Therefore, the general trend in recent years is toward the use of an IC testing apparatus having a handler called "horizontal transporting system" mounted thereto which can transport ICs to any desired place or position by using suction head means utilizing a vacuum pump which may pick up one to several ICs at a time and X and Y direction transfer means.
There have been previously used in practice following two types of IC testing apparatus each having a horizontal transporting system handler mounted thereto.
(1) One type of the IC testing apparatus is arranged such that a tray on which many ICs are loaded in a plane is placed at a predetermined position of the testing apparatus, a predetermined number of ICs are picked up by suction from the tray by use of a suction head utilizing a vacuum pump (vacuum suction head), the ICs being attracted against the vacuum suction head are transported to a test section through a preheating/precooling section by use of X and Y direction transfer means for testing, and upon completion of the test the tested ICs are sorted out into conformable articles (pass articles) and unconformable articles (failure articles), and transferred onto the corresponding trays by use of X and Y direction transfer means.
(2) The other type of the IC testing apparatus is arranged such that many ICs are loaded in a plane on a general-purpose tray (customer tray) which is used by a user for conveying ICs or storing ICs at a predetermined place or the like in the outside of the testing apparatus, the general-purpose tray with the ICs loaded is placed at a loader section of the testing apparatus where the ICs are transferred from the general-purpose tray onto a test tray capable of withstanding high/low temperatures, the test tray is transported through a constant temperature chamber or thermostatic chamber to a test section where ICs are brought into electrical contact with a tester head in the state that they are being loaded on the test tray for performing a test, and upon completion of the test the test tray with the tested ICs loaded are transported through a temperature-stress removing chamber to an unloader section where the tested ICs are sorted out into pass articles and failure articles and transferred onto the corresponding trays to be reloaded thereon.
The IC testing apparatus having a handler of the former type (1) mounted thereto has a disadvantage that since the number of ICs which undergo a test at a time is limited to two to four, the processing speed is low, and hence a considerable time is required to test all ICs. That is, the IC testing apparatus of the type (1) is not suitable for processing at high speed. On the other hand, the IC testing apparatus having a handler of the latter type (2) mounted thereto has an advantage that since ICs can be brought into electrical contact with a tester head of the testing apparatus in the state that they are being loaded on the test tray in the test section, it is possible to test many of ICs such as 16, 32 or 64 at a time. Therefore, at present, an IC testing apparatus having a handler of the latter type (2) mounted thereto is being mainly used.
A description will be given first regarding the general construction of a conventional IC testing apparatus having a handler of the latter type (2) mounted thereto with reference to FIGS. 4 and 5. The illustrated IC testing apparatus comprises a chamber section 100 for testing ICs such as semiconductor memories which are loaded on a test tray TST and carried on the test tray TST, an IC storage section 200 where ICs which will undergo a test (i.e., ICs to be tested) are sorted out and the tested ICs are sorted out and stored in place, a loader section 300 where ICs to be tested which a user has beforehand loaded on a general-purpose tray (customer tray) KST are transferred and reloaded onto a test tray TST capable of withstanding high/low temperatures, and an unloader section 400 where the tested ICs which have been carried on the test tray TST out of the chamber section 100 subsequently to undergoing a test in the testing chamber 100 are transferred from the test tray TST to one or more general-purpose trays KST to be reloaded on the latter. The unloader section 400 is generally constructed to sort out the tested ICs by categories on the basis of the data of the test results and load them on the corresponding general-purpose trays.
The chamber section 100 comprises a constant temperature or thermostatic chamber 100 for receiving the ICs to be tested loaded on the test tray TST and imposing an intended high or low temperature stress to the ICs, a test or testing chamber 102 for effecting an electrical test on the ICs subjected to the temperature stress in the constant temperature chamber 101, and a temperature-stress removing chamber 103 for removing the temperature stress of the ICs having been applied thereto in the test chamber 102 from the ICs. The test chamber 102 contains therein a tester head 104 of the testing apparatus, supplies various electric signals for testing via the tester head 104 to the ICs to be tested in electrically contact therewith, receives response signals from the ICs, and sends them to the testing apparatus.
Each of the test trays TST is moved in a circulating manner from the loader section 300 through the constant temperature chamber 101 of the chamber section 100, the test chamber 102 of the chamber section 100, the temperature-stress removing chamber 103 of the chamber section 100, and the unloader section 400 in this order, to the loader section 300. The constant temperature chamber 101 and the temperature-stress removing chamber 103 are taller than the test chamber 102, and have upward portions protruding beyond the top of the test chamber 102, respectively. As shown in FIG. 5, a base plate 105 spans between the upward protruding portions of the constant temperature chamber 101 and the temperature-stress removing chamber 103, and a test tray transporting means 108 is mounted on the base plate 105 to transport the test tray TST from the temperature-stress removing chamber 103 to the constant temperature chamber 101.
In case a temperature stress of a high temperature (a thermal stress) has been applied to the ICs to be tested in the constant temperature chamber 101, the temperature-stress removing chamber 103 cools the tested ICs down to room temperature by blowing, after which they are transported to the unloader section 400. On the other hand, in case a temperature stress of a low temperature such as, for instance, -30.degree. C. (a cryogenic stress) has been applied to the ICs to be tested in the constant temperature chamber 101, the temperature-stress removing chamber 103 heats the tested ICs by warm air or a heater up to a temperature at which the ICs have no any dew condensation, and then they are carried out of the temperature-stress removing chamber 103 to the unloader section 400.
The test tray TST with the ICs loaded thereon in the loader section 300 is conveyed from the loader section to the constant temperature chamber 101 within the chamber section 100. The constant temperature chamber 101 has a vertical conveyor means mounted therein which is adapted to support a plurality of (nine, for instance) test trays TST in the form of a stack. In the illustrated example, the vertical conveyor means stacks the transported test trays such that a test tray newly received from the loader section 300 is supported at the uppermost of the stack while the bottom test tray is delivered to the test chamber 102. The ICs to be tested on the uppermost test tray TST are given a predetermined high or low temperature stress while the associated test tray TST is moved sequentially from the top to the bottom of the stack by vertically downward movement of the vertical conveyor means and/or waits till the immediately preceding test tray is brought out of the test chamber 102. The tester head 104 is disposed in the test chamber 102 at the central area thereof, and each of the test trays TST carried out one by one from the constant temperature chamber 101 is conveyed onto the tester head 104 while maintained at the constant temperature, and a predetermined number of the ICs among the ICs on the associated test tray TST are electrically connected to IC sockets (not shown) mounted on the tester head 104, as will be discussed hereinbelow. Upon completion of the test on all of the ICs placed on one test tray TST through the tester head 104, the test tray TST is transported to the temperature-stress removing chamber 103 where the tested ICs on the associated test tray are relieved of temperature stress to be restored to the ambient or room temperature, and thereafter the test tray TST is discharged to the unloader section 400.
Like the constant temperature chamber 101 as described above, the temperature-stress removing chamber 103 is also equipped with a vertical conveyor means adapted to support a plurality of (nine, for instance) test trays TST stacked one on another. In the illustrated example, the test tray TST newly received from the test chamber 102 is supported at the bottom of the stack while the uppermost test tray is discharged to the unloader section 400. The tested ICs on the associated test tray are relieved of temperature stress to be restored to the outside temperature (room temperature) as the associated test tray TST is moved from the bottom to the top of the stack by vertically upward movement of the vertical conveyor means.
The tested ICs as carried on the test tray TST are passed to the unloader section 400 where they are sorted out by categories based on the test results and transferred from the test tray TST onto and stored in the corresponding general-purpose trays for respective categories. The test tray TST thus emptied in the unloader section 400 is transported to the loader section 300 where it is again loaded with ICs to be tested from a general-purpose tray KST onto the test tray TST, after which the same steps of above-described operation are repeated.
As shown in FIG. 5, an IC transfer means for transferring ICs from a general-purpose tray KST to a test tray TST in the loader section 300 may be in the form of X and Y direction transfer means 304 which comprises a pair of spaced parallel rails 301 mounted on the base plate 105 and extending over the loader section 400 in the front-to-back or forward-rearward direction of the testing apparatus (referred to as the Y direction herein), a movable arm 302 which spans between the two rails 301 and has its opposite ends secured thereto in a manner to be movable in the Y direction, and a movable head 303 which is supported by the movable arm 302 in a manner to be movable in the direction in which the movable arm 302 extends, that is in the left to right direction of the testing apparatus (referred to as the X direction herein). With this arrangement, the movable head 303 is allowed to reciprocate between the test tray TST and the general-purpose tray KST in the Y direction and move along the movable arm 302 in the X direction.
On the underside of the movable head 303 are vertically movably mounted IC suction pads. Through the movement of the movable head 303 in the X and Y directions and the downward movement of the suction pads in combination, the suction pads are brought into abutment with the ICs placed on the general- purpose tray KST and pick them up and hold thereto by vacuum suction to transfer them to the test tray TST. The number of suction pads that are mounted on the movable head 303 may be eight, for instance, so that a total of eight ICs may be transferred from the general-purpose tray KST to the test tray TST at one time.
It is to be noted here that means 305 for correcting the position of an IC called "preciser" (FIG. 5) is located between stopping positions for the general-purpose tray KST and the test tray TST. The position correcting means 305 includes relatively deep recesses into which the ICs as being attracted against the suction pads are once released to fall prior to being transferred to the test tray TST. The recesses are each defined by vertical tapered side walls which prescribe for the positions at which the ICs drop into the recesses by virtue of the tapering. After eight ICs have been precisely positioned relative to each other by the position correcting means 305, those eight ICs accurately positioned are again attracted against the suction pads and conveyed to the test tray TST. The reason that the position correcting means 305 is provided is as follows. Recesses of the general-purpose tray TST for holding the ICs are sized larger as compared to the size of ICs, resulting in wide variations in positions of ICs placed on the general-purpose tray KST. Consequently, if the ICs as such were vacuum picked up by the suction pads and transferred directly to the test tray TST, there might be some of the ICs which could not be successfully deposited into the IC storage recesses formed in the test tray TST. This is the reason for requiring the position correcting means 305, as described above which acts to array ICs as accurately as the array of the IC storage recesses formed in the test tray TST.
The unloader section 400 is equipped with two sets of X and Y direction transfer means 404 which are identical in construction to the X and Y direction transfer means 304 provided for the loader section 300. The X and Y direction transfer means 404 perform to transship the tested ICs from the test tray TST delivered out to the unloader section 400 onto the general-purpose tray KST. Each set of the X and Y direction transfer means 404 comprises a pair of spaced parallel rails 401 mounted to extend in the forward-rearward direction of the testing apparatus (Y direction), a movable arm 402 spanning between the pair of rails 401 and movably mounted at opposite ends on the pair of rails 401 in the Y direction, and a movable head 403 mounted on the movable arm 402 for movement therealong longitudinally of the arm, that is, in the right to left direction of the testing apparatus (X direction).
FIG. 6 shows the construction of one example of the test tray TST. The illustrated test tray TST comprises a rectangular frame 12 having a plurality of equally spaced apart parallel cleats 13 between the opposed side frame members 12a and 12b of the frame, each of the cleats 13 having a plurality of equally spaced apart mounting lugs 14 protruding therefrom on both sides thereof and each of the side frame members 12a, 12b opposing the adjacent cleats having similar mounting lugs 14 protruding therefrom. The mounting lugs 14 protruding from the opposed sides of each of the cleats 13 are arranged such that each of the mounting lugs 14 protruding from one side of the cleat 13 is positioned intermediate two adjacent mounting lugs 14 protruding from the opposite side of the cleat. Similarly, each of the mounting lugs 14 protruding from each of the side frame members 12a and 12b is positioned intermediate two adjacent mounting lugs 14 protruding from the opposed cleat. Formed between each pair of opposed cleats 13 and between each of the side frame members 12a and 12b and the opposed cleats are spaces for accommodating a multiplicity of IC carriers 16 in juxtaposition. More specifically, each IC carrier 16 is accommodated in one of an array of rectangular carrier compartments 15 defined in each of said spaces, each compartment 15 including two staggered, obliquely opposed mounting lugs 14 located at the diagonally opposed corners of the compartment. In the illustrated example wherein each cleat 13 has sixteen mounting lugs 14 on either side thereof, there are sixteen carrier compartments 15 formed in each of said spaces, in which sixteen IC carriers 16 are mounted. Since there are four of the spaces, 16.times.4, that is, 64 IC carriers in total can be mounted in one test tray TST. Each IC carrier 16 is placed on corresponding two mounting lugs 14 and fixed thereto by fasteners 17.
Each of IC carriers 16 is of identical shape and size in its outer contour and has an IC pocket 19 in the center for accommodating an IC element therein. The shape and size of the IC pocket 19 is determined depending on those of the IC element 18 to be accommodated therein. In the illustrated example, the IC pocket 19 is in the shape of a generally square recess. The outer dimensions of the IC pocket 19 are sized so as to be loosely fitted in the space defined between the opposed mounting lugs 14 in the carrier compartment 15. The IC carrier 16 has flanges at its opposed ends adapted to rest on the corresponding mounting lugs 14, these flanges having mounting holes 21 and holes 22 formed therethrough, respectively, the mounting holes 21 being adapted to receive fasteners 17 therethrough and the holes 22 being adapted to pass locating pins therethrough.
In order to prevent IC elements from slipping out of place within the IC carrier 16 or jumping out of the IC carrier 16, a pair of latches 23 are attached to the IC carrier 16, as shown in FIG. 7. These latches 23 are integrally formed with the body of the IC carrier so as to extend upwardly from the base of the IC pocket 19, and are normally resiliently biased such that the top end pawls are urged toward each other by virtue of the resiliency of the resin material of which the IC carrier is made. When the IC element is to be deposited into or removed from the IC pocket 19, the top ends of the two latches 23 are expanded away from each other by a latch releasing mechanism 25 disposed on opposite sides of an IC suction pad 24 for picking up an IC element prior to effectuating the deposition of the IC element into or removal from the IC pocket 19 upon the latch releasing mechanism 25 being moved out of engagement with the latches 23, the latches 23 will snap back to their normal positions by their resilient forces where the deposited IC is held in place against dislodgement by the top end pawls of the latches 23.
The IC carrier 16 holds an IC element in place with its leads or pins 18 exposed downwardly as shown in FIG. 8. The tester head 104 has an IC socket mounted thereto, and contacts 26 of the IC socket upwardly extend from the top surface of the tester head 104. The exposed leads 18 of the IC element are pushed against the contacts 26 of the IC socket to establish electrical connection between the IC element and the socket. To this end, a pusher 20 for pushing and holding an IC element down is mounted above the tester head 104 and is configured to push the IC element accommodated in an IC carrier 16 from above into contact with the tester head 104.
The number of IC elements which may be connected with the tester head 104 at a time depends on the number of IC sockets mounted on the tester head 104. By way of example, where sixty-four IC elements are arranged in an array of 4 lines .times.16 rows on a test tray TST as shown in FIG. 9, 4.times.4, that is, 16 IC sockets are arranged and mounted on the tester head 104 such that the IC elements (shown as obliquely hatched) in every fourth row in each of the lines may be tested all at one time. More specifically, in the first test run the examination is conducted on sixteen IC elements located in the first, fifth, ninth and thirteenth rows in each line, the second test run is effected on another sixteen IC elements located in the second, sixth, tenth and fourteenth rows in each line by shifting the test tray TST by a distance corresponding to one row of IC elements, and the third and fourth test runs are carried out in the similar manner until all of the IC elements are tested. The test results are stored in a memory at the addresses determined by, for instance, serial numbers (serial numbers in one lot or batch) assigned to ICs, the identification number given to the test tray TST and the numbers assigned to the IC pockets in the test tray. It is to be appreciated that where thirty-two IC sockets may be mounted on the tester head 104, only two test runs are required to examine all sixty-four IC elements arranged in an array of 4 lines.times.16 rows. It is also to be noted that there is another type of IC handler in which ICs to be tested are transferred from the test tray into a socket mounted on the tester head 104 and upon the test being completed the tested ICs are transferred from the socket back onto the test tray to transport the ICs, in the test chamber 102.
The IC storage section 200 comprises an IC storage rack (or stocker) 201 for accommodating general-purpose trays KST loaded with ICs to be tested and a tested IC storage rack (or stocker) 202 for accommodating general-purpose trays KST loaded with tested ICs sorted out by categories on the basis of the test results. The IC storage rack 201 and tested IC storage rack 202 are configured to accommodate general-purpose trays in the form of a stack. The general-purpose trays KST with ICs to be tested carried thereon and stored in the form of a stack in the IC storage rack 201 are transported successively from the top of the stack to the loader section 300 where the ICs to be tested (DUTs) are transferred from the general-purpose tray KST onto a test tray TST on standby in the loader section 300.
Each of the IC storage rack 201 and the tested IC storage rack 202 may be of identical shape and structure. Either of the IC storage rack 201 and any one of the tested IC storage racks 202 comprises, as any one of the IC storage rack 201 and the tested IC storage racks 202 is shown in FIG. 10, a tray supporting frame 203 open at the top and having an opening at the bottom, and an elevator 204 disposed below the frame 203 so as to be vertically movable through the bottom opening. In the tray supporting frame 203 there are stored and supported a plurality of general-purpose trays KST stacked one on another which are vertically moved by the elevator 204 acting through the bottom opening of the frame 203.
In the example illustrated in FIGS. 4 and 5, eight racks STK-1, STK-2, . . . , STK-8 are provided as tested IC storage racks 202 so as to be able to store tested ICs which may be sorted out into eight categories at a maximum according to the test results. This is because in some applications tested ICs may not only be classified into categories of "conformable or pass article" and "unconformable or failure article" but also be subclassified into those having high, medium and low operation speeds among the "pass" articles and those required to be retested among the "failure" articles, and others. Even if the number of classifiable categories is up to eight, the unloader section 400 in the illustrated example is capable of accommodating only four general-purpose trays KST. For this reason, if there occur some among the tested ICs which should be classified into a category other than categories assigned to the general-purpose trays KST arranged in the unloader section 400, the procedures taken are to return one of the general-purpose trays KST from the unloader section 400 to the IC storage section 200 and in replacement to transfer a general-purpose tray KST for storing the ICs belonging to the new additional category from the IC storage section 200 to the unloader section 400 where those ICs are stored in the new tray.
Referring to FIG. 5, a tray transfer means 205 is disposed above the IC storage rack 201 and the tested IC storage racks 202 for movement over the entire extent of the storage racks 201 and 202 in the direction of arrangement of the racks (in the right to left direction of the testing apparatus) relative to the base plate 105. The tray transfer means 205 is provided on its bottom with grasp means for grasping a general-purpose tray KST. The tray transfer means 205 is moved to a position over the IC storage rack 201 whereupon the elevator 204 is actuated to lift the general-purpose trays KST stacked in the IC storage rack 201, so that the uppermost general-purpose tray KST may be picked up by the grasp means of the tray transfer means 205. Once the uppermost general-purpose tray KST loaded with ICs to be tested has been transferred to the tray transfer means 205, the elevator 204 is lowered to its original position. The tray transfer means 205 is then horizontally moved to and stopped at a predetermined position in the loader section 300 where the grasp means of the tray transfer means 205 is released to allow the general-purpose tray KST to drop into an immediately underlying tray receiver (not shown). The tray transfer means 205 from which the general-purpose tray KST has been unloaded is moved out of the loader section 300. Then, the elevator 204 is moved upward from below the tray receiver having the general-purpose tray KST deposited thereon to lift up the tray receiver and hence the general-purpose tray KST loaded with ICs to be tested so that the general-purpose tray KST is kept exposed up through a window 106 formed in the base plate 105.
The base plate 105 is formed in the area overlying the unloader section 400 with another two similar windows 106 through which empty general-purpose trays are kept exposed. In this example, each of the windows 106 is sized to expose two general-purpose trays therethrough. Hence, four empty general-purpose trays are kept exposed up through two windows 106. Tested ICs are sorted out and stored in these empty general-purpose trays KST according to the categories assigned to respective trays. As with the loader section 300, the four empty general-purpose trays KST are placed on the respective tray receivers which are moved up and down by the associated elevators 204. Once one general-purpose tray KST has been fully filled, the tray is lowered from the level of the window 16 by the elevator 204 and stored in the tray storage position assigned to said tray by the tray transfer means 205. Indicated by the numeral 206 in FIGS. 4 and 5 is an empty tray storage rack for accommodating empty general-purpose trays KST. From this empty tray storage rack 206, empty general-purpose trays are transported to the respective windows 106 by the tray transfer means 205 and the elevators 204 and held thereat by the associated elevators 204 to be ready for receiving tested ICs.
As described above, in an IC testing apparatus having a handler of the foregoing type (2) mounted thereto in which ICs to be tested are transferred onto a test tray and transported to the test section (chamber section) to perform a test, it is possible to reduce a time required to test all the ICs because the number of ICs undergoing a test at a time can be increased. On the other hand, in the unloader section it takes a considerable time to carry out the transfer operation of the tested ICs since only ICs of eight or so are sorted out and transferred from a test tray onto a general-purpose tray at a time. Moreover, the transfer operation of the tested ICs in the unloader section accompanies the sorting operation of the tested ICs which takes a considerable time. To this end, though two sets of X and Y direction transfer means are provided in the unloader section 400, there still occurs a disadvantage that a time needed to sort out the tested ICs is longer than a time required to test all the ICs.
In addition, in an IC testing apparatus having a handler of the foregoing type (2) mounted thereto, in transferring the tested ICs from a test tray TST onto a general-purpose tray KST in the unloader section 400, the X and Y direction transfer means 404 stores in a storage device the facts that the tested ICs on the test tray have been transferred onto general-purpose trays by storing the addresses assigned to the respective IC carriers 16 on the associated test tray TST, and it performs the transfer operation of the tested ICs onto general-purpose trays on the basis of the stored addresses so as not to remain any tested IC or ICs which have failed to transfer on the test tray TST. However, there is a rare case that the tested IC or ICs remain on the test tray without being transferred therefrom.
If one or more tested ICs should not have been transferred and have remained on the test tray TST in the unloader section 400, the test tray TST loaded with one or more ICs not transferred is transported to the loader section 300, and hence an IC or ICs to be tested are loaded on the remaining tested IC or ICs in the form of a stack. In such case, the IC to be tested positioned at the upper side of the stack protrudes upwardly from the upper surface of the test tray. Therefore, there occurs a disadvantage that when the test tray loaded with the stack or stacks each of two ICs is transported to the constant temperature chamber 101 and then the subsequent test tray is stacked on the test tray with the stack or stacks of two ICs in the constant temperature chamber 101, the IC to be tested positioned at the upper side of the stack and protruding upwardly is pushed out of the associated test tray by insertion of the subsequent test tray and dropped down therefrom or an accident such as breakage of the IC to be tested may happen.
If an accident occurs that an IC drops down out of the associated test tray TST in the constant temperature chamber 101, it may occur that the IC drops down on a carrying mechanism or the like provided on the lower side of the constant temperature chamber 101 and interferes therewith so that the carrying mechanism can fail to convey. In addition, if the IC to be tested as being stacked on the remaining tested IC should be tested and transported to the unloader section 400 without dropping out of the test tray, the upper IC in the stack is sorted out on the basis of the test results of the lower remaining tested IC in the stack, and hence there is a disadvantage that an erroneous classification is done.